My opinion is that email addresses should never be used as usernames. Usernames, by definition, are public information.
Given that you allow email addresses as usernames on your site, you could a template override where instead of a username you get something like "User 123" where 123 is their user ID. This still suffers from the fact that you are leaking the user ID which needn't be public (even though it's not exactly private either).
Which leaves us with the last solution. A template override where non-manager users are simply displayed as "Client" when the person logged in is neither a manager, nor the user himself.
As for doing that as a config option, no, this won't happen. I am against config options for things which should be a template override. A litany of config options makes it very hard for us, the developers, to refactor the interface (e.g. whenever we will inevitably need to go to a newer version of Bootstrap), and it makes it extremely difficult for site integrators to make template overrides. The latter means that even more things need config options, which makes it ever harder for us to manager the interface etc. It creates a feedback loop which results in an unmaintainable interface.
This is not speculation, or me being a jerk; we've been there, and done that with Akeeba Subscriptions back in 2012. We said yes to every kind of config option which changes the interface. Within a year we could no longer maintain the interface. We needed a hard reset so badly, I had to do something I have never done since: discontinue a product without a replacement (every product I've discontinued since is because it's been included in Joomla! itself).
Nicholas K. Dionysopoulos
Lead Developer and Director
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